This invention relates to development systems for electrostatographic processors and, more particularly, to hybrid crossmixers for such systems.
In a conventional electrostatographic printing process of the type described in Carlson's U.S. Pat. No. 2,297,691 on "Electrophotography," a uniformly charged imaging surface is selectively discharged in an image configuration to provide a latent electrostatic image which is then developed through the application of a finely divided coloring material, called "toner." That process has enjoyed outstanding commercial success, especially in xerographic copiers and duplicators.
Xerography, of course, involves the use of a photoconductive imaging surface which is coated or otherwise deposited on an electrically conductive backing, such as a drum or an endless belt in a plain paper copier. There are, however, other types of electrostatographic processors. For example, there are some wherein the imaging surface is a uniformly charged electrically insulative member which is selectively discharged non-photographically -- e.g., by appropriately controlled stylii -- to provide a latent electrostatic image of the same general type as is photographically generated in a xerographic processor. Moreover, it should be understood that xerographic and similar electrostatographic processes are not limited to use in stand alone copiers and duplicators. For example, those processes have also been found to have utility in the facsimile art.
The vehicle normally used in electrostatographic processors to deliver the toner needed for development purposes is a multi-component developer comprising a mixture of toner particles and larger, so-called "carrier" particles. The materials for the toner and carrier (or, sometimes, carrier coating) components of the mixture are selected so that they are removed from each other in the triboelectric series, whereby electrical charges of opposite polarities tend to be imparted to the toner and carrier particles when they rub together. Furthermore, in making those selections, consideration is given to the triboelectric ranking of the materials to the end that the polarity of the charge nominally imparted to the toner particles opposes the polarity of the latent images which are to be developed. Consequently, in operation, there are competing electrostatic forces acting on the toner particles. Specifically, there are forces which at least initially tend to attract them to the carrier particles. Additionally, the toner particles are subject to being electrostatically stripped from the carrier particles whenever they are brought into the immediate proximity of or actual contact with an image bearing imaging surface.
Existing development systems commonly include a toner dispenser for adding additional toner to the developer supply from time-to-time so that its toner concentration remains at a suitably high level. Also, there typically is a crossmixer for mixing and blending the toner and carrier components of the developer so that the toner is more or less uniformly distributed throughout the developer and the triboelectric charging of the toner and carrier particles is promoted.
Surprisingly, available crossmixers still show a weakness when measured by their ability to carry out the mixing and blending functions simultaneously. As a general rule, passive crossmixers are effective mixing devices, but only marginally acceptable as blending devices. Active crossmixers on the other hand, normally are effective blending devices, but only marginally acceptable as mixing devices. As used herein, "mixing" refers to the distribution of the toner throughout the developer, and "blending" refers to the agitation of the developer which leads to the triboelectric charging of the toner and carrier particles.